1. Field of the Invention
This invention relates to an surface acoustic wave device, and more particularly to an surface acoustic wave device operative by a parametric amplifying effect.
2. Description of the Prior Art
An surface acoustic wave (elastic surface wave) amplifier is a solid-state amplifier which generally employs a piezoelectric semiconductor or a suitable combination of a semiconductor and a piezoelectric material, and attains a desired amplification effect by interaction between an surface acoustic wave (hereinafter referred to as "surface wave") and a semiconductor charge.
In a conventional surface wave amplifier, a piezoelectric material such as lithium niobate and a semiconductor are disposed oppositely so as to be spaced a distance of about 1,000 .ANG., and a d.c. voltage is applied to the semiconductor in a direction along the surface of the semiconductor which defines the region to accomplish an amplifying operation by the interaction between a semiconductor charge and a surface wave caused by such application of the d.c. voltage.
This conventional amplifier, however, has the defect that continuous wave operation cannot be obtained due to Joule heat generated by the d.c. current. It has another difficulty in integration because of its non-monolithic structure.
There have been other conventional devices than that of d.c. voltage-application type, which utilize a parametric amplification effect. One of them is so constructed that a semiconductor material such as Si is provided in the propagation path of travelling waves on a piezoelectric material through a conductive liquid and so operated that a surface signal wave and pumping wave (propagated wave) are propagated in the same direction at a portion of the piezoelectric material and a space capacitance nonlinearity at a portion of the semiconductor is utilized to attain amplification of the surface signal wave.
This device, however, has such disadvantages that since the frequency of the pumping wave is much higher than that of the surface signal wave, there is some difficulty in the preparation of transducers for generating the pumping waves, and that since necessary pumping power is increased due to generation of harmonic components of the pumping wave, there is caused some disadvantage in actual operation. It has further disadvantages in integration, too.
Another type of conventional parametric amplifier is so operated that with respect to a surface wave signal propagating on a piezoelectric material, an exciting voltage is applied to a metal electrode provided in the propagation path for exciting the piezoelectric material at the relevant portion, thereof, thereby amplifying the surface signal wave propagating through the piezoelectric material due to a nonlinear effect of the piezoelectric material.